The invention is of particular interest for lighting applications.
Wireless lighting control is gaining popularity together with the market growth of LED lighting. Wireless lighting control is realized by integrating a low power RF module into the lighting unit to enable wireless connectivity. For example, the recently launched Philips hue bulb brings wireless connectivity to an LED bulb by integrating a ZigBee module into the bulb. The bulb also contains a microcontroller to control both the RF module and the LED light source for intelligent light control.
FIG. 1 is a block diagram of an example of an LED light point with RF connectivity. The main power source 10 is a relatively high voltage (e.g., 20-30V) DC voltage source for the whole light point including LED light source 12, controller 14, RF module 16 (i.e., an RF transceiver 18 and an optional RF amplifier 20) and other control circuits 22 (such as logic circuits, MOSFET drivers, etc.). The power source can include an AC-DC converter if driven by the mains. The LED light source 12 receives power from the power source 10 via an LED driver 24.
Most control circuits in LED light points usually operate at a relatively low DC voltage such as 5 V (DC), and most controllers and RF modules operate at an even lower DC voltage such as 3.3 V (DC).
For the sake of power efficiency, a power supply 26 is used between the high voltage power source 10 and the controller 14 and other circuits. The function of the power supply 26 is to decrease the voltage to a proper level. The power supply 26 contains two components, a DC-DC converter 28 which converts the voltage from the power source to 5 V (DC), and a voltage regulator 30 which further decreases the voltage from the DC-DC converter to 3.3 V (DC).
The controller 14 and the RF transceiver 18 of the RF module 16 can be on a single chip. For example, ZigBee system on chip products are known containing a controller and a 2.4 GHz RF transceiver.
For light points such as an MR16 LED bulb, which usually has a metal heat sink covering almost the whole surface of the fixture and in which the light point is installed in a metal dome, the RF amplifier 20 is necessary to ensure adequate RF performance, such as increased transmitting power (or working distance) and it includes a low-noise amplifier for improved receiving sensitivity.
An important requirement for the power supply 26 is to provide enough current at a constant voltage (e.g., 3.3 V (DC)) to ensure the normal operation of the controller and RF module. There are three typical operation modes which consume different levels of current:
Non-RF mode: only the controller is active and no RF communication takes place. The operating current is for example less than 10 mA.
Receive mode: the RF module is in receiving operation. The operating current of the RF transceiver is for example in the range of 20-30 mA. The operating current of the RF amplifier is for example less than 5 mA. Therefore the total current requirement is 30-40 mA.
Transmit mode: the RF module is in transmitting operation. The operating current of the RF transceiver is for example in the range of 30-40 mA. The operating current of the RF amplifier is for example in the range of 80-180 mA. Therefore the total current requirement is 120-240 mA.
Clearly the transmission operation requires significantly higher current than the other operations. Another feature of the transmission operation is that it is a periodic operation with low frequency and extremely short duration. For example, on average every 5-15 seconds there is a transmission with a duration of less than 10 milliseconds. In other words, most of the time the controller and RF module consumes 10-40 mA, but demands high peak (120-240 mA) transient currents during the periodic transmit operation.
There are two commonly adopted approaches for designing the power supply 26 for wireless connected LED light points.
The first approach is to use a DC-DC converter with high current supply capacity and fast transient response to accommodate the high peak transient current demand from the RF module when transmitting. The main disadvantage is that such converters are bulky and expensive, which limits the usage in applications where compact space and low-cost are mandatory.
The second approach is to use a normal DC-DC converter together with a large value reservoir capacitor or electrolytic capacitor. The converter can tolerate the receiving operation of the RF module, while the capacitor can help to absorb the transient peak currents during the transmission operation. This approach has a lower cost compared with the first approach, but a large value (e.g., in the range of 100-200 μF) capacitor is still bulky.
Considering the need to enable wireless connectivity for LED light points with compact size (such as MR16 or MR11 bulbs), a low-cost and compact power supply solution is desired.
U.S. Pat. No. 5,422,562A1, US20060022653A1 and “Linear-nonlinear control applied to switching power supplies to get fast transient response”, 10.1109/IECON.2002.1187556 disclose that a second power supply can provide power to a load in case the power from a first power supply to the load is not enough.